1. Field of the Invention
This invention relates to an upper feed mechanism of a sewing machine, and more particularly to an upper feed mechanism of a sewing machine including an upper feed dog for making upper, lower, right and left motions by means of a main shaft, and feeding a cloth in cooperation with a lower feed dog.
2. Description of the Prior Art
The upper feed mechanism of a sewing machine is available in various types. One type comprises a crank oscillating back and forth by means of a main shaft, an upper feed bar of which a base part is rotatably mounted on the crank to oscillate up and down, having an upper feed dog opposing a lower feed dog disposed at a free end of the upper feed bar, a spring device for operating to push down the upper feed bar, and another crank oscillating vertically by means of the main shaft and moving the upper feed bar up and down in cooperation with the spring device. Another type comprises a crank oscillating back and forth by means of a main shaft, another crank oscillating up and down by means of the main shaft, an upper feed bar of which one end is coupled with the crank for horizontal feed by way of a first link and other end is provided with an upper feed dog opposing a lower feed dog, a second link coupling the middle part of the upper feed bar and the crank for vertical feed, a lever rotatably pivoted on the sewing machine frame, a third link coupling the lever and the pivotal part joining the first link and upper feed bar, and a pressure spring intervening between the lever and the sewing machine frame (see Japanese Laid-open Patent No. 60-227795). Moreover, as for the upper feed dog, a type of feeding upward as being fitted and disposed at the presser foot before the needle drop unit (hereinafter called front upper feed), and a type of feeding upward as being disposed at the rear side of the presser foot behind the needle drop point (rear upper feed), and the former front upper feed is suited to sewing a thin fabric which is easily deviated or subjected to ruffling, while the latter rear upper feed is suited to sewing denim or other thick and heavy fabric and is capable of feeding the cloth smoothly.
The conventional upper feed mechanism of a sewing machine also usually comprises an adjusting device for the upper feed amount within the mechanism for moving the crank for horizontal feed back and forth by means of the main shaft. This adjusting device is, in most cases, composed of an arc-shaped lever, a slider being slidably mounted on this lever and positioned by the operation of an operating mechanism, a crank for oscillating the lever, and a link for coupling the crank and slider, and it is composed so that the pivotal point of the link and crank may be matched with the center of the curvature of the lever, and the upper feed amount is designed to be changed while the rear position is constant in the front upper feed type or the front position is constant in the rear upper feed type.
A different version of the upper feed mechanism of a sewing machine comprises, for example, an adjusting mechanism for the vertical motion within a mechanism for moving the crank for vertical feed up and down by means of the main shaft, so that the vertical motion of the upper feed dog may be adjusted depending on the thickness of the fabric or stepped part (see Japanese Laid-open Utility Model No. 58-169862).
This invention is intended to exchange the upper feed dog for front upper feed and the upper feed dog for rear upper feed depending on the application so as to use the front upper feed and rear upper feed separately, but in the conventional upper feed mechanism if only the upper feed dogs were exchanged, the upper feed dog might collide with the presser foot when the upper feed amount was changed. In other words, in the sewing machine of front upper feed, if the upper feed dog is merely changed in position to behind the needle drop point, the rear position is constant and the upper feed amount changes, and therefore if the upper feed amount is increased, the upper feed dog may collide against the presser foot if moved to the front side. In the sewing machine of a rear upper feed type, similarly, when the upper feed amount is increased while the front position is constant and the upper feed amount is changed, the upper feed dog may collide with the presser foot upon completion of the cloth feed.